> In practice, I would like to know how simulators such as Hyperlynx account > for this C_comp? Does it exclude double-counting of the C_comp when > performing rise/fall time domain simulations? Lengthy discussions about this have come up on these maillists a few times before. Honestly, I don't see what the problem is, why people think that there is "double-counting" if a non-zero C_comp is included. The IBIS model of an actively driving device is very simple. It starts with just a switch, that changes in an ideal way from one state to the other, with some drive strength (the I-V curves) in each state. The I-V curves do not contain any information about C_comp in them. None whatsoever. I-V curves are static (DC) parameters, and are unaffected by C_comp. The I-V curves could be generated with C_comp=0, or C_comp=2pF, or C_comp=10,000uF, and it would make no difference to the I-V curves. C_comp is something that needs to be added to help make this model more representative, more accurate. The I-V curves represent the static drive strengths of the output transistors. C_comp represents the capacitance that is really there on the chip, in those transistors and around them. If the simulator didn't include C_comp in the simulation, the IBIS model would represent output transistors that had no capacitance, and they would switch incorrectly. You need C_comp to be there. C_comp is even more important when the driver tri-states and the IC receives signals from elsewhere. But I don't think anyone has an issue with "double-counting" C_comp when the device is a receiver. If the simulator primarily uses the I-V curves to create the driven waveforms from an output driver (and I believe that they do), then I think there's no issue with double-counting. The simulator needs to know about C_comp in order to adjust the dynamic response from the "perfect" switch that behaves according to those I-V curves. IBIS data sheets can also have V-T curves, and those curves were measured with the actual C_comp in the device (or in the SPICE model from which the IBIS data was extracted). What does the simulator do with the V-T data? As far as I know, they generally use it to adjust the simulator's transition from one state to the other (from one I-V curve to the other), but they do not use it to actually generate the driven waveforms themselves. That is, the simulator doesn't synthesize a time-dependent source whose value equals the V-T curve, and then attach C_comp to it. If it did, it would always be wrong. So simulators don't do that. If a simulator tries to replicate the V-T curve, it must know that it can only replicate it with C_comp present and with R_fixture present. Including them is not double-counting; it is providing exactly what the simulator expects to be there. Regards, Andy |------------------------------------------------------------------ |For help or to subscribe/unsubscribe, email majordomo@eda.org |with just the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or email a written request to ibis-request@eda.org. | |IBIS reflector archives exist under: | | http://www.eda.org/pub/ibis/email_archive/ Recent | http://www.eda.org/pub/ibis/users_archive/ Recent | http://www.eda.org/pub/ibis/email/ E-mail since 1993Received on Thu Mar 3 08:04:18 2005
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